1. Field of the Invention
The present invention relates generally to a method of stress relieving thick aluminum alloy plates (particularly thick plates of at least about 5″) exhibiting high mechanical properties, which allows reduction in the level of residual stress through the thickness of the plate, which in turn, reduces distortion after machining.
2. Description of Related Art
Thick plates are generally heat-treated to achieve high mechanical properties. Known processes include a solutionizing treatment at high temperature, followed by a cooling step, followed by a stress-relieving step. It is also known that stretching along the longest direction of a solution heat-treated and quenched aluminum plate may decrease the residual stress of the plate.
The article “Numerical Calculation of Residual-Stress Relaxation in Quenched Plates” by J. C. Boyer and M. Boivin (Material Science and Technology, October 1985, vol. 1, p. 786-753) includes theoretical calculations, which suggest that compression in the thickness direction of quenched plates in AA7075 alloy may decrease their residual stress. This is confirmed in the article, “A Finite Element Calculation of Residual Stresses After Quenching and Compression Stress Relieving of High Strength Aluminum Alloys Forgings,” by P. Jeanmart, B. Dubost, J. Bouvaist and M. P. Charue (published in Conference Residual Stresses in Science and Technology, vol. 2, p. 587-594 (DGM 1987)) on the basis of experimental results obtained on test cylinders in AA7010 alloy, and in the article, “Relief of Residual Stress in a High-Strength Aluminum Alloy by Cold Working,” by Y. Altschuler, T. Kaatz and B. Cina (published in “Mechanical Relaxation of Residual Stress”, ASTM STP 993, L. Mordfin, Ed., American Society for Testing and Materials, Philadelphia, 1988, p. 19-29) on the basis of measurement on specimens compressed in the thickness direction.
Since the mid-1990s, quenched plate in 7xxx alloys that have been stress-relieved by compression in the thickness direction (followed by aging to the T 7452 temper) are being used for the manufacture of certain structural components in aircrafts (see the article “Residual Stress in 7050 Aluminum Alloy Restruck Forged Block,” by T. Bains, published in the Proceedings of the 1st International Non-Ferrous Processing and Technology Conference, 10-12 Mar. 1997, St. Louis, p. 233-236). This process of compression in the thickness direction has been thoroughly investigated, especially in relation with subsequent aging treatments to T7542 temper. The influence of compression on aging response of AA7050 plate has been analyzed in a recent publication entitled, “On the Residual Stress Control in Aluminum Alloy 7050,” by K. Escobar, B. Gonzalez, J. Ortiz, P. Nguyen, D. Bowden, J. Foyos, J. Ogren, E. W. Lee and O. S. Es-Said (Materials Science Forum, Vols. 396-402, p. 1235-1240 (2002)). According to N. Yoshihara and Y. Hino's calculation and experimental evidence (“Removal Technique of Residual Stress in 7075 Aluminum Alloy”, ICRS Residual Stress III, Science and Technology vol. 2, p. 1140-1145 (1992)), compression (T7353) is more effective to relieve residual stress in small 7075 alloy blocks than the so-called uphill quench process (referenced as T7353).
U.S. Pat. Nos. 6,159,315 and 6,406,567 B1 (both assigned to Corus Aluminum Walzprodukte GmbH) disclose methods of stress relieving solution heat-treated and quenched aluminum alloy plates that include a combination of a stress-relieving cold mechanical stretch and a stress-relieving cold-compression, the cold stretch being performed in the length direction, and the cold-compression being performed in the thickness direction.